Turbine efficiency is desirable, for example, in large installations where a fractional increase in efficiency can result in a significant reduction in the amount of fuel that is needed to produce electrical power. This can lead to cost savings and lower emissions of CO2, with corresponding reductions of SOx and NOx.
Known turbine blades have been of aerofoil cross-section, a fixed blade extending radially between inner and outer end blocks and being of prismatic form. Orientations of both fixed and moving blades about their respective blade axes have also been standardised for the prismatic blade design. The orientation has been defined by the blade stagger angle between the turbine axial direction and a line tangential to blade leading edge and trailing edge circles on a pressure surface of the aerofoil blade.
A known improvement in the performance of the prismatic blade in the turbine has been attempted by imposing a “lean” on the blade, for example, tilting the blade about its root in a circumferential plane, (e.g., a plane transverse, or perpendicular, to the turbine axis). This “lean” can produce a variation in a mass flow at an outlet of the blade from a platform region (at a blade root) to a tip region.
Because the circumferential spacing of the blades (e.g., pitch) increases progressively from the platform region to the tip region, the position where a throat line intersects a suction surface moves upstream with increased radius. Due to the convex curvature of the suction surface, this can lead to an increase in the outlet angle from about 13° at the root (relative to the tangential direction) to about 15° at the tip.
A controlled flow aerofoil blade, which can provide a performance improvement over these known aerofoil blade designs, has been proposed in EP-B1-0704602.